Monoclinic M' yttrium tantalate x-ray phosphors are used in x-ray intensifying screens for medical radiographic applications. The x-ray screens are used in combination with x-ray films to produce x-ray images. Yttrium tantalate, YTaO.sub.4, based phosphors are desirable for use in x-ray screens because they have a higher x-ray stopping power than other conventional x-ray phosphors and the major emission peak from YTaO.sub.4, approximately 340 nm, overlaps favorably with the sensitive regions of certain commercial x-ray films. Examples of these phosphors are given in U.S. Pat. Nos. 5,009,807, 5,112,524, and 4,225,653 which are incorporated herein by reference.
One property associated with x-ray phosphors which can cause serious problems is the presence of delayed fluorescence. Delayed fluorescence is the emission of light from the phosphor after x-ray excitation stops. Persistence is the amount of delayed fluorescence exhibited by x-ray phosphor powders. A high persistence x-ray phosphor can compromise the quality of radiographic images collected using an x-ray screen made with that phosphor. This is a particular concern for x-ray intensifier screens used in automated changers for routine x-ray procedures. Because an intensifier screen in an automated changer is used many times over a short period of time, a high persistence phosphor can cause the screen to retain some of the previous image which will interfere with subsequent x-ray images taken with the same screen. Thus, it has become increasingly important to manufacture x-ray phosphors having the lowest possible persistence levels in order to obtain high quality radiographic images. Unfortunately, monoclinic M' yttrium tantalate phosphors commonly are afflicted with high persistence levels.
Thus, it would be an advantage to have monoclinic M' yttrium tantalate x-ray phosphors having reduced levels of persistence.